Wednesday, January 29, 2014

New paper finds no evidence of a change in the water cycle from AGW

A paper published today in Nature Climate Change finds that there is no evidence of a strengthening of the water cycle [increased evaporation] as predicted by climate models of anthropogenic global warming, instead finding "recent multi-year declines in global average continental evaporation" are related to transitions of the natural El Niño–La Niña cycle [ENSO].

The hydrological cycle is expected to intensify in response to global warming1, 2, 3. Yet, little unequivocal evidence of such an acceleration has been found on a global scale4, 5,6. This holds in particular for terrestrial evaporation, the crucial return flow of water from land to atmosphere7. Here we use satellite observations to reveal that continental evaporation has increased in northern latitudes, at rates consistent with expectations derived from temperature trends. However, at the global scale, the dynamics of the El Niño/Southern Oscillation (ENSO) have dominated the multi-decadal variability. During El Niño, limitations in terrestrial moisture supply result in vegetation water stress and reduced evaporation in eastern and central Australia, southern Africa and eastern South America. The opposite situation occurs during La Niña. Our results suggest that recent multi-year declines in global average continental evaporation8, 9 reflect transitions to El Niño conditions, and are not the consequence of a persistent reorganization of the terrestrial water cycle. Future changes in continental evaporation will be determined by the response of ENSO to changes in global radiative forcing, which still remains highly uncertain10, 11.

Excerpt:

Despite recent reports of negative trends in wind speed12, 13, the atmospheric demand for water vapour is expected to rise as the concentrations of greenhouse gases increase. This expectation follows from the physics of the Clausius–Clapeyron relation: the capacity of air to carry moisture grows with air temperature. Unsurprisingly therefore, most climate models predict an increase in global evaporation, and the subsequent intensification of the hydrological cycle. However, terrestrial evaporation (or evapotranspiration, E) also depends on the availability of water on land; soil moisture deficits reduce the rates of vaporization from soil and vegetation below the potential values that reflect atmospheric conditions alone14, 15, 16. Thus, the ability of climate models to predict future changes in the terrestrial water cycle relies on our understanding of the interactions between soil moisture, vegetation activity and E, and how these variables are affected by the climatic factors that control their long-term dynamics.

Until 2008, continental-scale observational data of E were not available7, 17 and changes in the terrestrial water cycle were diagnosed exclusively from trends in precipitation and river runoff 4, 18. Recent progress in deriving global E from satellite and in situ data9, 19 now allows for observation-based evaluation of global trends in E. Nonetheless, E cannot be directly sensed from satellites, and present methodologies must combine observable variables that play a role in the evaporation process. Some are fully statistical8, 20; others are more physically based17, 21, 22. These methodologies have recently estimated prolonged declines in global E, contradicting the expected acceleration of the water cycle8, 9. In particular, ref. 8 suggested a negative trend in E for 1998–2008, which was attributed to a concurrent reduction in soil water availability in the Southern Hemisphere. Whether such a regional-scale reduction might be a permanent feature of global warming or resulted from internal [natural] climate variability was left as an open question.

Our results, based on three decades of satellite observations, suggest that ENSO regulates the total volume of water vapour moving from continental land surfaces into the atmosphere. Terrestrial water shortages—as a consequence of low moisture supplies during El Niño—are the cause of vegetation stress and prolonged declines in E. Although the effects of ENSO are mainly confined to water-limited regions in the 30°N–60°S domain, they dominate the interannual variability of continental evaporation (Supplementary Fig. 9). This suggests that recent negative trends in continental evaporation8, 9 are largely caused by internal climate variability and do not constitute a persistent reorganization of the terrestrial water cycle. However, in the possible scenario of ‘El Niño-like’ conditions intensifying in the future10, 11, the total flux of water vapour from land to atmosphere may in fact be progressively reduced. A recent study predicts a more intense El Niño-driven drying in the western Pacific Ocean by the mid-twenty-first century30. This would affect Australia, where we show the highest sensitivity of transpiration and vegetation greenness to El Niño conditions. Consequences may be in the form of regional reductions in net primary production, water resources or ecosystem services, and the intensification of feedbacks from soil moisture on precipitation and temperature14, 31.